Thought Rosetta had it tough? Scientists are about to attempt an even more daring space landing

Fly, land, return: this is the summer of asteroid sample return missions. Japan’s Hayabusa2 has finally arrived in orbit around Ryugu after travelling for three and a half years, while US’s OSIRIS-Rex is on target to arrive at Bennu in August. So much interest in asteroids is totally warranted: not only they are time capsules from the formation of our solar system but these two are also carbon-rich and near Earth with orbits closer than Mars. Understanding them will help scientists piece together how planets form and tease future plans for asteroid mining.

But they are fairly small – four times smaller, in fact, than the comet 67P/Churyumov–Gerasimenko that the intrepid Rosetta probe and its lander, Philae, arrived at in 2014. Ryugu is about 865 meters in diameter, spinning like a top for days just seven and a half hours long. Bennu is under 500 meters, roughly twice the width of the river Thames at London Bridge, with an even shorter day of just over four hours. So how do you land a spacecraft on one? With great difficulty.

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Getting into orbit around an asteroid is a slow, patient game dictated by the physics of orbital dynamics. “You know the orbit of the asteroid, you know the orbit of the Earth, you know the rotation of the Earth, and then you can calculate exactly when you need to leave the surface of our planet to get onto that path,” explains says Dante Lauretta, principle investigator for OSIRIS-Rex and member of the Hayabusa2 team.

“You've got to be formation-flying with the asteroid, so you kind of sneak up on it while moving in the same direction,” he adds. It’s akin to a pair of cars racing down a highway together. “To pass something from one car to another, you've got to get right next to it and match its speed so that compared to the car, it doesn't appear to be moving,” says Lauretta. “Just reach across and hand over something to the other passenger.” Every spacecraft needs to accomplish this task of leaving Earth and matching speeds with its destination.

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Microgravity complicating things

But operating in the microgravity environment of an asteroid has unique challenges. There’s radiation pressure from the sun, outgassing from the spacecraft, thermal radiation and heat coming off the asteroid – all of this can have almost as much of an effect on the spacecraft as gravity, says Lauretta. “It's very complicated because we've got all these small forces that are kind of buffeting us around.”

Hayabusa2 is now precisely in this messy, complex region, orbiting just 20km above the surface of Ryugu as mission scientists frantically try to characterise the surface and locate the safest, most interesting place to touch down. “We want to sample pristine regolith, and do that while we keep the spacecraft safe,” says Lucille Le Corre, research scientist with the Hayabusa2 mission who is also working on the OSIRIS-Rex mission.

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“I think the most difficult thing for touchdown is where to land,” says Makoto Yoshikawa, Hayabusa2 mission manager. “If you look at the image of Ryugu, you can find a lot of boulders on the surface. We must look for the smooth surface to touchdown. Maybe this is very difficult for this case.”

Microgravity means that these space boulders - essentially piles of gravel loosely held together - don’t behave the same way we’d expect them to on Earth. “If you were a human out on an asteroid and you stuck your arm down into the pebbles, you'd pull your arm out and it would be covered in pebbles like you'd stuck your hand into a jar of powdered sugar or flour here on Earth,” says Alessondra Springmann, planetary scientist at the University of Arizona.

Collecting the samples

Then it needs to get the samples from the surface - the spacecraft isn’t just going to land and scoop up some material. Hayabusa2 “has a kinetic impactor, which is what we say in polite company when we don't want to say the word ‘bomb’,” says Springmann. This impactor will make a small crater of two to three metres in diameter on the surface of Ryugu. Then the craft will very delicately swoop down to the crater, collect the samples, “and then book it out of there and hopefully back to Earth,” she adds.

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The idea is to bring the spacecraft early next year, with two more possible touchdowns in the following months before departing for Earth late in 2019.

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This isn’t Japan’s first attempt at an asteroid sampling mission. The first Hayabusa rendezvoused with asteroid Itokawa in 2005, successfully delivering 1,500 particles back to Earth – during a mission where everything that could possibly go wrong did. The spacecraft’s sampling procedure glitched during both landing attempts. “Hayabusa landed twice: the first landing was a failure because it was an emergency landing and the second one was okay, but the projectile was not shot,” says Yoshikawa. This time, with Hayabusa2, he hopes to land without any issues.

To do it flawlessly, many interactions have to be scripted and rehearsed ahead of time, explains Springmann – due to time lag over such great distances, the controllers may not be able to do much if something starts to go wrong.

Going back home

Hayabusa2 didn’t arrive to Ryugu alone – but escorted by a “mini-fleet” of smaller spacecraft that will make several hops across the surface taking measurements at different locations, says Springmann. The German-built MASCOT lander will study the asteroid’s surface composition and properties, while the three MINERVA landers, successors to a failed lander that accompanied the original Hayabusa, will collect images and surface temperatures.

If everything goes well, after 18 months studying the asteroid, Hayabusa2 will head back to Earth with precious samples of Ryugu onboard. When close to home, a sample return capsule will separate from the spacecraft, using heat shields and parachutes to burn off the speed of reentry from falling down Earth’s gravity well.

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With over 700,000 known asteroids and more of them being discovered all the time, scientists usually use telescopes to study them. But interpreting properties of an object just by looking at it as a point of light or an occasional meteorite is limited. So asteroid sampling missions are essential to test the models, assumptions and properties inferred via telescopes or from meteorites. When meteorites fall on the Earth, they get mostly destroyed. What’s not destroyed gets “eroded pretty quickly, and the composition is degraded,” says Le Corre. Asteroid sampling missions are essential to test the models, assumptions and inferred properties inferred via telescopes or from meteorites.

Ryugu and Bennu are carbonaceous, meaning they are carbon-rich – and the material they are made of hasn't changed much since the solar system formed 4.6 billion years ago. The samples will allow scientists to investigate organic molecules from the very early solar system, potentially giving insight into the origins of life.

While both Hayabusa2 and OSIRIS-Rex are sample return missions, Lauretta says it’s not a competition. The agencies have already agreed to share any samples that are successfully returned to Earth. If one mission fails, everyone still has samples to work on. If they both succeed, they get a greater diversity of samples. “In some ways we're each other's insurance policy,” says Lauretta, adding that “each asteroid is its own unique world and worthy of exploration. Getting the samples from them will really help us piece together the geologic history of our solar system.”